In digital image processing, an image is typically represented as a number of pixels. Each pixel's color is defined by the color's coordinates in some color space, e.g. sRGB. The display converts the color coordinates to “grey levels” which are then used to define electrical signals (e.g. voltages) that determine luminous states of corresponding areas on the screen of the display. Sometimes, the color coordinates themselves can be used as grey levels. Prior to display, these grey levels are usually adjusted by some function (usually called a “gamma function”, “gamma transfer function”, “gamma transfer characteristic”, or “gamma curve”).
For many displays, the gamma function is non-linear, and can be approximated by a power relationship:L=xγ  (1)where L is the normalized luminance, x is the grey level, and γ is a constant for the display. In many CRTs (Cathode Ray Tubes), LCDs (Liquid Crystal Displays), and some other types of devices, γ is about 2.2. However, the relationship (1) and/or γ value are approximate, and can vary. Such variation can be adjusted or corrected for by, for example, using look-up tables (LUTs) tabulating values for a specific relationship that may differ somewhat from (1).
Therefore, with reference to FIG. 1, conventional systems often apply a single gamma function such as (1) to input image data (e.g., sRGB data received from a transmission) as part of some current widely-used image display standards, e.g. to correctly display sRGB images. More specifically, as shown in FIG. 1, conventional systems employ a single Panel Gamma block that receives digital input image data (typically, 8-bit digital image data intended for display, such as output from a graphics card), applies gamma function (1) to the image data and converts it to analog, and outputs the gamma-corrected, analog luminance values that drive a display.
A typical color display is associated with a set of primary colors, e.g. red, green and blue. The display accepts separate grey levels for each of the primary colors (i.e. for each “channel”). The gamma functions can be different for different channels, and therefore separate LUTs can be provided for each channel.
For example, a color LCD may include a number of red, green and blue subpixels. The subpixels have identical liquid crystal cells, but have color filters of different colors (red, green, blue). The liquid crystal cells however have different optical activity with respect to wavelength and hence to color. Depending on the spectral bandwidth of the color filters, such optical activity can result in unequal luminance gamma transfer characteristics amongst the R, G, B channels. In addition, the optical activity can result in chrominance deviations (e.g. hue deviations) within each channel. The unequal gamma transfer characteristics can be corrected using the separate LUTs for each channel.